1. Field of the Invention
The present invention relates to woven preforms for reinforced composite materials and, in particular, to multidirectionally reinforced shape woven preforms for composite structures.
2. Incorporation by Reference
All patents, patent applications, documents, references, manufacturer's instructions, descriptions, product specifications, and product sheets for any products mentioned herein are incorporated herein by reference, and may be employed in the practice of the invention.
3. Description of the Prior Art
The use of reinforced composite materials to produce structural components is now widespread, particularly in applications where their desirable characteristics are sought of being light in weight, strong, tough, thermally resistant, self-supporting and adaptable to being formed and shaped. Such components are used, for example, in aeronautical, aerospace, satellite, recreational (as in racing boats and autos), and other applications.
Typically such components consist of reinforcement materials embedded in matrix materials. The reinforcement component may be made from materials such as glass, carbon, ceramic, aramid, polyethylene, and/or other materials which exhibit desired physical, thermal, chemical and/or other properties, chief among which is great strength against stress failure. Through the use of such reinforcement materials, which ultimately become a constituent element of the completed component, the desired characteristics of the reinforcement materials, such as very high strength, are imparted to the completed composite component. The constituent reinforcement materials typically, may be woven, knitted, nonwoven or otherwise oriented into desired configurations and shapes for reinforcement preforms. Usually particular attention is paid to ensure the optimum utilization of the properties for which the constituent reinforcing materials have been selected. Usually such reinforcement preforms are combined with matrix material to form desired finished components or to produce working stock for the ultimate production of finished components.
After the desired reinforcement preform has been constructed, matrix material may be introduced to and into the preform, so that typically the reinforcement preform becomes encased in the matrix material and matrix material fills the interstitial areas between the constituent elements of the reinforcement preform. The matrix material may be any of a wide variety of materials, such as epoxy, phenolic, polyester, vinyl-ester, ceramic, carbon and/or other materials, which also exhibit desired physical, thermal, chemical and/or other properties. The materials chosen for use as the matrix may or may not be the same as that of the reinforcement preform and may or may not have comparable physical, chemical, thermal or other properties. Typically, however, they will not be of the same materials or have comparable physical, chemical, thermal or other properties, since a usual objective sought in using composites in the first place is to achieve a combination of characteristics in the finished product that is not attainable through the use of one constituent material alone. So combined, the reinforcement preform and the matrix material may then be cured and stabilized in the same operation by thermosetting or other known methods, and then subjected to other operations toward producing the desired component. It is significant to note at this point that after being so cured, the then solidified mass of the matrix material normally is very strongly adhered to the reinforcing material (e.g., the reinforcement preform). As a result, stress on the finished component, particularly via its matrix material acting as an adhesive between fibers, may be effectively transferred to and borne by the constituent material of the reinforcement preform.
Frequently, it is desired to produce components in configurations that are other than such simple geometric shapes as (per se) plates, sheets, rectangular or square solids, etc. A way to do this is to combine such basic geometric shapes into the desired more complex forms. One such typical combination is made by joining reinforcement preforms made as described above at an angle (typically a right-angle) with respect to each, other. Usual purposes for such angular arrangements of joined reinforcement preforms are to create a desired shape to form a reinforcement preform that includes one or more end walls, or to strengthen the resulting combination of reinforcement preforms and the composite structure which it produces against deflection or failure upon it being exposed to exterior forces, such as pressure or tension. In any case, a related consideration is to make each juncture between the constituent components as strong as possible. Given the desired very high strength of the reinforcement preform constituents per se, weakness of the juncture becomes, effectively, a “weak link” in a structural “chain”.
In related art, U.S. Pat. No. 4,847,063 is taught a hollow, carbon/carbon composite article around an axis of symmetry, in which a heat refractory mandrel is shaped to provide a desired inner surface configuration, and one or more plies of thermally stable carbon filaments are braided over the surface of the mandrel. The plies are then impregnated with a carbonizable liquid impregnant, and the assembly of impregnated plies and mandrel is heat treated so as to carbonize the impregnant.
U.S. Pat. No. 5,070,914 relates to a triaxial textile fabric for use as a reinforcing textile fabric for a composite material wherein the modulus of elasticity is made isotropic and the fabric is deformed into a three-dimensional configuration without changing the orientation angles. The fabric comprises a large number of oblique yarns extending in a radial direction from the center of the textile fabric, and a circumferential yarn woven spirally in a circumferential direction between the oblique yarns. Each of the adjacent oblique yarns is interlaced with each other and the circumferential yarn is woven between the interlaced oblique yarns such that such interlacing appears between each adjacent coil of the spirally woven circumferential yarn. The interlacing step takes place after insertion of the circumferential yarn and before an upward and downward movement of the alternate oblique yarns.
U.S. Pat. No. 5,619,903 teaches a braided preform for composite structures, having a longitudinal axis and a plurality of braided strands of structural fiber. An elongated member, a pultruded rod, having a rigidity greater than that of the strands of structural fiber is intertwined into the braided strands parallel to the longitudinal axis of the braided member, which is a triaxially braided tube.
While the prior art has sought to improve upon the structural integrity of the reinforced composite and has achieved success, there exists a desire to improve thereon or address the problem through an approach different from those disclosed above. In this regard, one approach might be by creating a woven three-dimensional (“3D”) structure by specialized machines. Another approach would be to weave a two dimensional (“2D”) structure and fold it into shape. However, this typically results in parts that distort when the preform is folded. The distortion occurs because the lengths of fiber as-woven are different than what they should be when the preform is folded. This causes dimples and ripples in areas where the as-woven fiber lengths are too short, and buckles in the areas where fiber lengths are too long. These distortions cause undesirable surface anomalies and reduce the strength and stiffness of the component. While this may be relieved by cutting and darting, such a procedure is undesirable, since it is labor intensive or otherwise may compromise the integrity of the preform. Axisymmetric 3D preforms are therefore desirable because they provide increased strength relative to 2D laminated composites. These preforms are particularly useful in applications that require the composite to carry out-of-plane loads, such as in aerospace applications.
Other techniques include filament winding, which is a continuous fiber placement technique whereby fibers are wrapped around a mandrel. This fabrication technique is not applicable to all geometrical shapes. For example, fiber bridging occurs along surfaces that have concave shapes. Furthermore, it is difficult to obtain a uniform coverage of material in a curved section using filament winding. Although filament winding is a highly automated technology, filament winding a complex shape multi-layer laminate can require considerable time due to the low material dispensing rate.
Tow placement is a technology similar to filament winding in that single/multiple tows or ribbons of material are dispensed onto a mandrel. Unlike filament winding, the tows can be discontinuous in length and tows can be placed along concave surfaces. Using discontinuous tows, it is possible to obtain near uniform coverage of material across a curved part and it is possible to maintain the desired fiber angle. However, the process can be slow and restricted to preimpregnated materials, and therefore expensive.
U.S. Pat. No. 5,394,906 (hereinafter merely, “the '906 patent”) relates to an apparatus for fabricating straight or curved planar or three-dimensional fabrics for fabrication into composite structures. The fill yarns are inserted between layers of warp yarns, and a canted or curved reed, depending on the orientation of the fill yarns, is used to compact or “beat-up” the fill yarns. The '906 patent further discloses that the warp yarns of the fabric can be curved using a conical or a combination of conical and cylindrical rollers to effect differential fabric take-up of the warp yarns for obtaining a constant radius of curvature of the warp yarns. In a further embodiment, the '906 patent discloses a clamping bar fabric take-up device used to effect the differential fabric take-up of the warp yarns for obtaining straight warp yarns, curved warp yarns with a constant radius of curvature, curved warp yarns with a non-constant radius of curvature, or some combination of straight and curved warp yarns. In yet another embodiment, flanges for a curved ‘C’ shape are formed by alternately inserting the warp yarns through adjacent dent wires of the reed to permit vertical weaving of the flanges. These features, however, require custom designed weaving machines that use canted or curved reeds to insert fill yarns between the warp yarns. The machine also requires a special design for its “beat-up” mechanism, which again depends on the orientation of the fill yarns in the structure. The production of a special weaving machine merely to produce a particular design structure is not only expensive, but also is confined to that particular design. Additionally, these machines run at speeds relatively slower than the conventional weaving machine, because the insertion of the fill yarn follows a canted or curved path, which drastically reduces the loom speed.
U.S. Pat. No. 6,086,968 (hereinafter merely, “the '968 patent”) provides woven materials having a variety of two- and three-dimensional shapes. The materials are constructed by continuously varying the density and/or directions of the warp and/or weft fibers or yarns at will during the mechanical weaving process. These materials are woven around an object and are impregnated thereafter with matrix material.
While the '906 and '968 patents offer advantages over other prior art discussed above, for many structural applications that are not truly axisymmetric, additional structural features need to be added to reinforce the structure and provide mechanical stability in all directions. Accordingly, mere use of conventional 2D or 3D woven preforms does not adequately provide the required strength in all directions. Therefore, it would be an advancement in the state of the art to provide a structure and method of forming multidirectionally reinforced shaped woven preforms for composite structures that have improved strength in both axisymmetric as well as non-axisymmetric configurations.